Return correct type definition for queries

[JeanMertz]

see: #6353 (comment)

Given this VRL program:

foo = [{ "foo": to_string!(1) }]
bar = foo[0].foo
upcase(bar)

This should be an infallible program, as we know the variable bar always resolves to a string.

[JeanMertz]

Tested this in the CLI on master, and this is still an issue that needs to be solved:

foo = [{ "foo": to_string(1) }]
bar = foo[0].foo
upcase(bar)

error[E110]: invalid argument type
  ┌─ :1:8
  │
1 │ upcase(bar)
  │        ^^^
  │        │
  │        this expression resolves to the exact type "null"
  │        but the parameter "value" expects the exact type "string"
  │
  = see language documentation at: https://vector.dev/docs/reference/vrl/
  = learn more at: https://errors.vrl.dev/110